1. Field of the Invention
This invention relates generally to systems and devices for sensing or measuring the motion of fluids and producing an electrical signal indicative of such motion.
2. Description of the Related Art
There are many situations where it is necessary or desirable to sense or measure the velocity, pressure, or flow rate of a fluid. Such situations include fluids that are both liquids and gasses and include both internal and external flow. For example, such systems or devices may be used to measure the air-speed of an aircraft, the water flow rate in a canal, pipeline, or river, the air flow rate into an internal combustion engine, or the flow rate through a ventilation duct in a heating, ventilating, and air-conditioning (HVAC) system of an office building. In many such applications, such as where electronic controls are used, it is necessary or desirable for such measuring devices to provide an electronic output that can be displayed on an electronic display or directly interface with an electronic control system. Many such systems and devices have been developed over the years including hot wire systems and systems and devices that measure velocity or flow rate indirectly by measuring pressure or pressure differences. Systems and devices that measure velocity or flow rate indirectly by measuring pressure or pressure differences include those that use structures that extend into the flow stream such as pitot tubes, and those that measure from the side of the flow stream such as venturi meters.
Some flow measurement systems or devices impede the flow to some degree, such as weirs and orifice plates, while others, such as pitot tubes, typically do not impede the flow significantly. In some applications, some degree of obstruction of the flow is not a problem, such as a weir wall used to measure the flow rate in a stream. However, in other applications, it is necessary or desirable to impede the flow as little as possible, such as applications where significant amounts of energy are required to generate the flow. Where the flow rate of a fluid is the desired parameter to be measured, many systems and devices measure the flow rate by measuring the velocity at one or more locations, and using the velocity to calculate the flow rate. The flow rate may be either a volumetric flow rate or a mass flow rate.
Historically, systems for measuring the motion of fluids, such as flow sensors, have been stand-alone systems of their own. They have been bulky, expensive, quite large and not suitable for integration into small electronic systems. There has also been a lack of flexibility in design preventing incorporation into a variety of subsystems. More recent use of thick film hybrid type airflow sensing products have greatly reduced size and improved integration potential using single in-line electrical connections. They also have electrical input and output requirements more compatible with typical electronic products. Some such products have the additional advantage of being manifold mountable, which reduces system space requirements and allows semi-automation of the pneumatic part of product assembly. While these examples are improvements over prior art, they still require manual electronic assembly at high cost and low throughput. In addition, their size is still typically the largest component in a control system circuit. There is a clear lack of a small sensor, particularly for airflow, that can be assembled by automated equipment and handled, as are other electronic components.
Previous solutions have included alternative sensing techniques, like differential pressure sensors, which typically suffered from poor accuracy. The other sensors have problems of their own, being expensive and of limited availability in standard package configurations.
It is an object of this invention to provide a system for measuring the motion of a fluid, the system having a maximum of features and capable of being manufactured in a very small standard package. Other objects of the invention are to provide an improved system that will integrate control systems, lower cost, reduce power consumption, and reduce size while implementing highly automated mass assembly manufacturing. Further objects of the invention include that the product be compatible with factory automation, that it have improved interchangeability, that it have a minimum number of parts and that it be readily capable of being modified for manifold mount, minimum pressure drop or controlled pressure drop applications.
In furtherance of these objects, the present invention provides a system for sensing the motion of a fluid that has a body with an internal flow passage, an electronic sensor located within the flow passage, and a metal lead frame that is in electrical communication with the sensor and integrally molded with the body. The body may be made of two parts that are joined by a joint that may snap together or attach with an adhesive, heat staking, or ultrasonic welding. The body may have outside dimensions of width, depth, and height that are each less than 1.5 inches, or may have a volume of less than 0.25 cubic inch. The body may be made of plastic or thermal plastic, and the metal of the lead frame may have a lower coefficient of thermal expansion than the plastic to reduce leakage between the two. Specifically, the metal may have a coefficient of thermal expansion that is less than 10 parts per million per degree Celsius and the plastic may have a coefficient of thermal expansion that is greater than 50 parts per million per degree Celsius. The body may have two elongated port tubes configured to attach to tubing, and the port tubes may each have a venturi. The sensor may have an RTD and a heat source. In one embodiment, the internal flow passage and the sensor are substantially symmetrical and the system is configured to measure the flow rate of the fluid substantially equally in either flow direction. In this embodiment, the sensor may comprise two RTDs located on either side of a heat source. The fluid sensed with the present invention may be a gas, such as air. The system may be configured for surface mounting or for through-hole mounting. The motion that is sensed may be flow rate, or specifically, mass flow rate. The lead frame may have pins on substantially opposite sides. The body may have pins from the lead frame projecting from one side that are all in substantially the same plane, or may have pins from the lead frame projecting from two sides where the pins from each side are in substantially the same plane. The system may also comprise a control system for a heating, ventilating, and air-conditioning system, or even the heating, ventilating, and air-conditioning system itself.
The present invention also provides a system for sensing the pressure or flow rate of a fluid with a body that has an internal flow passage and is formed from two parts that are joined by a joint that has an elastomeric seal that is integral with one part. There is also a transducer located within the flow passage. The two parts of the body may be attachable to each other with snap connectors, and the body may have outside dimensions of width, depth, and height that are each less than 1.5 inches. The body may have a port tube, which may have a venturi, and may be substantially straight. The system may be a dual in-line package.
The present invention further provides a system for measuring the flow rate of a gas with a body that has an internal flow passage and is formed in two parts that are configured to snap together. There is also a sensor located within the flow passage. The body may have outside dimensions of width, depth, and height such that width multiplied by depth multiplied by height is less than 0.5 cubic inch.